Spc. Neftaly Platero was charged with two counts of murder and one count of attempted murder in a Sept. 23 shooting in Fallujah, about 40 miles (65 kilometers) west of Baghdad.
A statement issued late Wednesday by the U.S. military in Baghdad described the killings as premeditated but offered no other details.
Earlier, a U.S. military spokesman said Platero was accused of shooting the three soldiers after a verbal altercation. The spokesman, Col. Barry Johnson, did not know what the argument was about.
Pfc. Gebrah Noonan, 26, of Watertown, Connecticut, and Spc. John Carillo Jr., 20, of Stockton, California, died the day after the shooting. The name of the wounded soldier has not been released, and the statement Wednesday said the criminal investigation of the incident is ongoing.
Platero, 32, from Houston, Texas, is being held in pretrial confinement in Kuwait. He will be tried by a military court.
He was assigned to Camp Ramadi, in western Iraq, with the 3rd Infantry Division based out of Fort Stewart, Georgia.
Meanwhile, four gunmen stormed the home of a policeman in northern Iraq, killing him and four members of his family. An Iraqi police official said three women were among the dead.
The late Wednesday raid occurred in the northern city of Mosul, located about 225 miles (360 kilometers) northwest of Baghdad. Mosul is no longer considered an al-Qaida stronghold as violence around Iraq has ebbed. But police and other security officials remain prime targets for insurgents.
An official at Mosul Medical Hospital confirmed the death toll in the raid. Both officials spoke on condition of anonymity because they are not allowed to talk to the media.
Associated Press Writer Lara Jakes in Baghdad contributed to this report.